Obesity is rapidly on the rise and is associated with dysregulated metabolism, cardiovascular morbidity and increased rates of gastrointestinal malignancies. Moreover, obesity is driven by high-fat, high-cholesterol diets that are themselves serious risk factors for gastrointestinal cancers. The most rapidly-dividing cells in the body reside in the intestinal epithelium, which relies on progenitor cells like intestinal stem cells (ISCs) to sustain it. While dietary lipids are known to regulate ISCs, the mechanisms by which lipids affect intestinal proliferation are not well understood. Lipid biosynthesis is regulated by sterol regulatory element-binding proteins (SREBPs). My recent studies have demonstrated that SREBPs also have a critical role in sustaining growth of the intestinal epithelia. I found, surprisingly, that a deficiency of SREBP-2, which blocks cholesterol synthesis, results in marked intestinal overgrowth and increased numbers of intestinal progenitor cells. I hypothesized that this increase in proliferatory capacity of the intestine was caused by a compensatory stimulation of SREBP-1 in the SREBP-2 knockouts. Next, I found that overexpression of SREBP-1 in intestine reproduced this intestinal overgrowth, confirming that SREBP-1, which controls fatty acid synthesis, is a key regulator of intestinal epithelial growth. The goal of this proposal is to determine the molecular mechanisms by which SREBPs controls intestinal epithelial proliferation. The central hypothesis is that SREBPs maintain homeostasis of intestinal epithelia by producing lipid metabolites that sustain proliferation of intestinal progenitors. Thus, the following specific aims will be pursued: (1) To determine how SREBP-1 drives the growth of intestinal epithelia. (2) To identify the cellular compartment(s) where SREBPs mediate their effects on intestinal growth. (3) Using unbiased transcriptome and lipidome profiling, to identify the lipid metabolites and their biosynthetic enzymes that underlie the effects of SREBPs on the epithelium. These aims will be achieved with novel mouse models using Cre-loxP tools to modulate gene and marker expression in intestine in vivo, RNA-seq and lipidomics measurements, and intestinal organoids for in vitro mechanistic experiments. The expected outcome of this proposal is the identification of mechanisms by which SREBPs regulates intestinal growth. This outcome will have positive translational impact because it is feasible that the newly-identified lipid regulators will represent novel therapeutic targets that could be manipulated pharmacologically. Growth stimuli could be inhibited in the setting of intestinal cancers for anti-cancer effect or stimulated in the setting of short bowel. Lastly, a better understanding of intestinal lipid metabolism in the intestine will facilitate the development of new treatments for complications of obesity like NAFLD, which is epidemic in the U.S.